Processes for making tissue products, such as tissue and towel, are well known. Soft, absorbent disposable tissue products, such as facial tissue, bath tissue and tissue toweling, are a pervasive feature of contemporary life in modem industrialized societies. While there are numerous methods for manufacturing such products, in general terms, their manufacture begins with the formation of a cellulosic fibrous web in the forming section of a tissue making machine. The cellulosic fibrous web is formed by depositing fibrous slurry, that is, an aqueous dispersion of cellulosic fibers, onto a moving forming fabric in the forming section of a tissue making machine. A large amount of water is drained from the slurry through the forming fabric, leaving the cellulosic fibrous web on the surface of the forming fabric. Further processing and drying of the cellulosic fibrous web generally proceeds using at least one of two well-known methods.
These methods are commonly referred to as wet-pressing and drying. In wet pressing, the newly formed cellulosic fibrous web is transferred to a press fabric and proceeds from the forming section to a press section that includes at least one press nip. The cellulosic fibrous web passes through the press nip(s) supported by the press fabric, or, as is often the case, between two such press fabrics. In the press nip(s), the cellulosic fibrous web is subjected to compressive forces which squeeze water therefrom. The water is accepted by the press fabric or fabrics and, ideally, does not return to the fibrous web or tissue.
After pressing, the tissue is transferred, by way of, for example, a press fabric, to a rotating Yankee dryer cylinder that is heated, thereby causing the tissue to substantially dry on the cylinder surface. The moisture within the web as it is laid on the Yankee dryer cylinder surface causes the web to adhere to the surface, and, in the production of tissue and towel type products, the web is typically creped from the dryer surface with a creping blade. The creped web can be further processed by, for example, passing through a calendar and wound up prior to further converting operations. The action of the creping blade on the tissue is known to cause a portion of the interfiber bonds within the tissue to be broken up by the mechanical smashing action of the blade against the web as it is being driven into the blade. However, fairly strong interfiber bonds are formed between the cellulosic fibers during the drying of the moisture from the web. The strength of these bonds is such that, even after conventional creping, the web retains a perceived feeling of hardness, a fairly high density, and low bulk and water absorbency. In order to reduce the strength of the interfiber bonds that are formed by the wet-pressing method, Through Air Drying (“TAD”) can be used. In the TAD process, the newly formed cellulosic fibrous web is transferred to a TAD fabric by means of an air flow, brought about by vacuum or suction, which deflects the web and forces it to conform, at least in part, to the topography of the TAD fabric. Downstream from the transfer point, the web, carried on the TAD fabric, passes through and around the Through-Air-Dryer, where a flow of heated air, directed against the web and through the TAD fabric, dries the web to a desired degree. Finally, downstream from the Through-Air-Dryer, the web may be transferred to the surface of a Yankee dryer for further and complete drying. The fully dried web is then removed from the surface of the Yankee dryer with a doctor blade, which foreshortens or crepes the web thereby further increasing its bulk. The foreshortened web is then wound onto rolls for subsequent processing, including packaging into a form suitable for shipment to and purchase by consumers.
As noted above, there are multiple methods for manufacturing bulk tissue products, and the foregoing description should be understood to be an outline of the general steps shared by some of the methods. Further, there are processes that are alternatives to the Through-Air-Drying process that attempt to achieve “TAD-like” tissue or towel product properties without the TAD units and high energy costs associated with the TAD process.
The properties of bulk, absorbency, strength, softness, and aesthetic appearance are important for many products when used for their intended purpose, particularly when the fibrous cellulosic products are facial or toilet tissue or towels. To produce a tissue product having these characteristics on a tissue making machine, a woven fabric will be used that is often constructed such that the sheet contact surface exhibits topographical variations. These topographical variations are often measured as plane differences between woven yarn strands in the surface of the fabric. For example, a plane difference is typically measured as the difference in height between a raised weft or warp yam strand or as the difference in height between machine-direction (MD) knuckles and cross-machine direction (CD) knuckles in the plane of the fabric's surface
In some tissue making processes as mentioned above, an aqueous nascent web is initially formed in the forming section from a cellulose content furnish, using one or more forming fabrics. Transferring the formed and partly dewatered web to the press section, comprising one or more press nips and one or more press fabrics, the web is further dewatered by an applied compressive force in the nip. In some tissue making machines, after this press dewatering stage, a shape or three dimensional texture is imparted to the web, with the web thereby being referred to as a structured sheet. One manner of imparting a shape to the web involves the use of a creping operation while the web is still in a semi-solid, moldable state. A creping operation uses a creping structure such as a belt or a structuring fabric, and the creping operation occurs under pressure in a creping nip, with the web being forced into openings in the creping structure in the nip. Subsequent to the creping operation, a vacuum may also be used to further draw the web into the openings in the creping structure. After the shaping operation(s) are complete, the web is dried to substantially remove any desired remaining water using well-known equipment, for example, a Yankee dryer.
There are different configurations of structuring fabrics and belts known in the art. Specific examples of belts and structuring fabrics that can be used for creping in a tissue making process can be seen in U.S. Pat. Nos. 7,815,768 and 8,454,800 which are incorporated herein by reference in their entirety.
Structuring fabrics or belts have many properties that make them conducive for use in a creping operation. In particular, woven structuring fabrics made from polymeric materials, such as polyethylene terephthalate (PET), are strong, dimensionally stable, and have a three dimensional texture due to the weave pattern and the spaces between the yarns that make up the woven structure. Fabrics, therefore, can provide both a strong and flexible creping structure that can withstand the stresses and forces during use on the tissue making machine The openings in the structuring fabric, into which the web is drawn during shaping, can be formed as spaces between the woven yarns. More specifically, the openings can be formed in a three dimensional manner as there are “knuckles” or crossovers of the woven yarns in a specific desired pattern in both the machine direction (MD) and cross machine direction (CD). As such, there is an inherently limited variety of openings that can be constructed for a structuring fabric. Further, the very nature of a fabric being a woven structure made up of yarns effectively limits the maximum size and possible shapes of the openings that can be formed. Thus, while woven structuring fabrics are structurally well suited for creping in tissue making processes in terms of strength, durability and flexibility, there are limitations on the types of shaping to the tissue making web that can be achieved when using woven structuring fabrics. As a result, there are limits to simultaneously achieving higher caliper and higher softness of a tissue or towel product made using a woven fabric for the creping operation.
As an alternative to a woven structuring fabric, an extruded polymeric belt structure can be used as the web-shaping surface in a creping operation. Openings (or holes or voids) of different sizes and different shapes can be formed in these extruded polymeric structures, for example, by laser drilling, mechanical punching, embossing, molding, or any other means suitable for the purpose.
The removal of material from the extruded polymeric belt structure in forming the openings, however, has the effect of reducing the strength and resistance to both MD stretch and creep, as well as durability of the belt. Thus, there is a practical limit on the size and/or density of the openings that may be formed in an extruded polymeric belt while still having the belt be viable for a tissue making creping process.
One requirement of a creping belt or fabric is to be configured to substantially prevent cellulose fibers in the web of the tissue or towel product from passing through the openings of the creping belt in the creping nip. As a result, sheet properties such as caliper, strength and appearance will be less than optimum.